Documentation: dmaengine: document DMA_CTRL_ACK

Add documentation about acking the transfers, and their
reusability.

Signed-off-by: Robert Jarzmik <robert.jarzmik@free.fr>
Acked-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>